1. Field of the Invention
This invention relates to a surface treating method and apparatus wherein the surface of substrates with an axially symmetric shape such as bars, tubes or fibers can be treated by an atmospheric pressure plasma technique uniformly and efficiently.
2. Description of the Prior Art
Several surface treating or surface property-improved methods are known in the art wherein substrate surfaces are treated to impart hydrophilic or water-repellent properties thereto. Among them, treatment by discharge techniques such as corona discharge treatments, low pressure glow plasma treatments and the like are important as a dry and clean treating method.
The corona discharge treatment is effected at an atmospheric pressure in an atmosphere of air or nitrogen and is the most inexpensive and simplest treating technique among discharge treatments. However, since electrons are discharged in the form of a beam for the corona discharge, the treated surface is liable to become uneven. In addition, the gas temperature tend to be significantly higher than a gas temperature in a low temperature plasma, thus presenting the problem that the surface treatment other than a hydrophilic treatment is very difficult. The degree of hydrophilicity attained by the corona discharge treatment is usually poorer than in the case of plasma treatments.
On the other hand, according to the low pressure glow plasma treatment, it is possible to effect a uniform surface treatment with a reduced degree of unevenness, with the further possibility that a variety of highly functional surfaces are obtained by treatments wherein the type of atmospheric gas, input electric power, frequency and pressure are appropriately controlled. However, since the low pressure plasma treatment is generally performed at a pressure as low as not higher than 10 Torr., a large-scale vacuum apparatus is necessary for carrying out the treatment on an industrial scale, resulting in an increase of equipment costs and treating costs. In addition, if a material to be treated contains moisture or gaseous plasticizers in large amounts, these are vaporized under reduced pressure conditions and are caused to be discharged from the surface of the material to be treated. This may result in properties and functions which are not intended for the plasma treatment. Moreover, such a plasma treatment as set out above is liable to generate heat during the course of the treatment, with an attendant problem that a material to be treated which is made of low melting materials is difficult to treat.
Another surface treating technique which solve the problems of both types of treating techniques has been proposed, in which a stable glow plasma is obtained at an atmospheric pressure and is used to treat the surface of a substrate (Japanese Laid-Open Patent Application Nos. 1-306569 and 2-15171).
The surface treatment by the atmospheric pressure plasma technique is carried out at a pressure close to an atmospheric pressure, so that a large-scale vacuum apparatus is not necessary and this technique is appropriately applicable to substrates which contain large amounts of moisture or gas-emitting plasticizers. In addition, little heat is generated at the time of the treatment. Accordingly, this treatment is applicable to low melting substrates and also to a local portion, i.e. only a predetermined portion of a substrate may be treated.
The atmospheric pressure plasma treating apparatus set forth in the above-indicated applications makes use of parallel flat plate electrodes or similarly shaped electrodes. Attempts have been made to the shape of the electrode surface in order to obtain a uniform discharge at an atmospheric pressure. However, our investigations reveal that the surface of an axially symmetric substrate such as bars, tubes or fibers are difficult to be uniformly, efficiently treated according to the methods of the applications.